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1.
ACS Nano ; 13(10): 11460-11468, 2019 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-31502825

RESUMO

Periodic arrays of strongly coupled colloidal quantum dots (QDs) may enable unprecedented control of electronic band structure through manipulation of QD size, shape, composition, spacing, and assembly geometry. This includes the possibilities of precisely engineered bandgaps and charge carrier mobilities, as well as remarkable behaviors such as metal-insulator transitions, massless carriers, and topological states. However, experimental realization of these theoretically predicted electronic structures is presently limited by structural disorder. Here, we use aberration-corrected scanning transmission electron microscopy to precisely quantify the orientational disorder of epitaxially connected QD films. In spite of coherent atomic connectivity between nearest neighbor QDs, we find misalignment persists with a standard deviation of 1.9°, resulting in significant bending strain localized to the adjoining necks. We observe and quantify a range of out-of-plane particle orientations over thousands of QDs and correlate the in-plane and out-of-plane misalignments, finding QDs misoriented out-of-plane display a statistically greater misalignment with respect to their in-plane neighbors as well. Using the bond orientational order metric ψ4, we characterize the 4-fold symmetry and introduce a quantification of the local superlattice (SL) orientation. This enables direct comparison between local orientational order in the SL and atomic lattice (AL). We find significantly larger variations in the SL orientation and a statistically robust but locally highly variable correlation between the orientations of the two differently scaled lattices. Distinct AL and SL behaviors are observed about a grain boundary, with a sharp boundary in the AL orientations, but a more smooth transition in the SL, facilitated by lattice deformation between the neighboring grains. Coupling between the AL and SL is a fundamental driver of film growth, and these results suggest nontrivial underlying mechanics, implying that simplified models of epitaxial attachment may be insufficient to understand QD growth and disorder when oriented attachment and superlattice growth occur in concert.

2.
Nano Lett ; 19(7): 4797-4803, 2019 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-31199150

RESUMO

Aliovalent doping of CdSe nanocrystals (NCs) via cation exchange processes has resulted in interesting and novel observations for the optical and electronic properties of the NCs. However, despite over a decade of study, these observations have largely gone unexplained, partially due to an inability to precisely characterize the physical properties of the doped NCs. Here, electrostatic force microscopy was used to determine the static charge on individual, cation-doped CdSe NCs in order to investigate their net charge as a function of added cations. While the NC charge was relatively insensitive to the relative amount of doped cation per NC, there was a remarkable and unexpected correlation between the average NC charge and PL intensity, for all dopant cations introduced. We conclude that the changes in PL intensity, as tracked also by changes in NC charge, are likely a consequence of changes in the NC radiative rate caused by symmetry breaking of the electronic states of the nominally spherical NC due to the Coulombic potential introduced by ionized cations.

3.
Mol Microbiol ; 109(6): 812-825, 2018 09.
Artigo em Inglês | MEDLINE | ID: mdl-29995992

RESUMO

The size of whole Rhodobacter sphaeroides prevents 3D visualization of centermost chromatophores in their native environment. This study combines cryo-focused ion beam milling with cryo-electron tomography to probe vesicle architecture both in situ and in 3D. Developing chromatophores are membrane-bound buds that remain in topological continuity with the cytoplasmic membrane and detach into vesicles when mature. Mature chromatophores closest to the cell wall are typically isolated vesicles, whereas centermost chromatophores are either linked to neighboring chromatophores or contain smaller, budding structures. Isolated chromatophores comprised a minority of centermost chromatophores. Connections between vesicles in growing bacteria are through ~10 nm-long, ~5 nm-wide linkers, and are thus physical rather than functional in terms of converting photons to ATP. In cells in the stationary phase, chromatophores fuse with neighboring vesicles, lose their spherical structure, and greatly increase in volume. The fusion and morphological changes seen in older bacteria are likely a consequence of the aging process, and are not representative of connectivity in healthy R. sphaeroides. Our results suggest that chromatophores can adopt either isolated or connected morphologies within a single bacterium. Revealing the organization of chromatophore vesicles throughout the cell is an important step in understanding the photosynthetic mechanisms in R. sphaeroides.


Assuntos
Cromatóforos Bacterianos/ultraestrutura , Rhodobacter sphaeroides/ultraestrutura , Membrana Celular/metabolismo , Microscopia Crioeletrônica , Tomografia com Microscopia Eletrônica , Fotossíntese/fisiologia
4.
Ultramicroscopy ; 191: 56-65, 2018 08.
Artigo em Inglês | MEDLINE | ID: mdl-29843097

RESUMO

Combining multiple fast image acquisitions to mitigate scan noise and drift artifacts has proven essential for picometer precision, quantitative analysis of atomic resolution scanning transmission electron microscopy (STEM) data. For very low signal-to-noise ratio (SNR) image stacks - frequently required for undistorted imaging at liquid nitrogen temperatures - image registration is particularly delicate, and standard approaches may either fail, or produce subtly specious reconstructed lattice images. We present an approach which effectively registers and averages image stacks which are challenging due to their low-SNR and propensity for unit cell misalignments. Registering all possible image pairs in a multi-image stack leads to significant information surplus. In combination with a simple physical picture of stage drift, this enables identification of incorrect image registrations, and determination of the optimal image shifts from the complete set of relative shifts. We demonstrate the effectiveness of our approach on experimental, cryogenic STEM datasets, highlighting subtle artifacts endemic to low-SNR lattice images and how they can be avoided. High-SNR average images with information transfer out to 0.72 Å are achieved at 300 kV and with the sample cooled to near liquid nitrogen temperature.

5.
Proc Natl Acad Sci U S A ; 115(7): 1445-1450, 2018 02 13.
Artigo em Inglês | MEDLINE | ID: mdl-29382750

RESUMO

Incommensurate charge order in hole-doped oxides is intertwined with exotic phenomena such as colossal magnetoresistance, high-temperature superconductivity, and electronic nematicity. Here, we map, at atomic resolution, the nature of incommensurate charge-lattice order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature ([Formula: see text]93 K). In diffraction, the ordering wave vector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the ordered state forms lattice-locked regions over a few wavelengths interspersed with phase defects and changing periodicity. The cations undergo picometer-scale ([Formula: see text]6 pm to 11 pm) transverse displacements, suggesting that charge-lattice coupling is strong. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93 K. Such local phase variations govern the long-range correlations of the charge-ordered state and locally change the periodicity of the modulations, resulting in wave vector shifts in reciprocal space. These atomically resolved observations underscore the importance of lattice coupling and phase inhomogeneity, and provide a microscopic explanation for putative "incommensurate" order in hole-doped oxides.

6.
J Am Chem Soc ; 140(10): 3652-3662, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29376343

RESUMO

Magic-sized clusters (MSCs) are renowned for their identical size and closed-shell stability that inhibit conventional nanoparticle (NP) growth processes. Though MSCs have been of increasing interest, understanding the reaction pathways toward their nucleation and stabilization is an outstanding issue. In this work, we demonstrate that high concentration synthesis (1000 mM) promotes a well-defined reaction pathway to form high-purity MSCs (>99.9%). The MSCs are resistant to typical growth and dissolution processes. On the basis of insights from in situ X-ray scattering analysis, we attribute this stability to the accompanying production of a large (>100 nm grain size), hexagonal organic-inorganic mesophase that arrests growth of the MSCs and prevents NP growth. At intermediate concentrations (500 mM), the MSC mesophase forms, but is unstable, resulting in NP growth at the expense of the assemblies. These results provide an alternate explanation for the high stability of MSCs. Whereas the conventional mantra has been that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing), we demonstrate that anisotropic clusters can also be stabilized by self-forming fibrous mesophase assemblies. At lower concentration (<200 mM or >16 acid-to-metal), MSCs are further destabilized and NPs formation dominates that of MSCs. Overall, the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions. This work provides not only a robust method to synthesize, stabilize, and study identical MSC products but also uncovers an underappreciated stabilizing interaction between surfactants and clusters.

7.
Nat Commun ; 8(1): 1883, 2017 12 01.
Artigo em Inglês | MEDLINE | ID: mdl-29192204

RESUMO

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal-insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.

8.
ACS Appl Mater Interfaces ; 9(15): 13500-13507, 2017 Apr 19.
Artigo em Inglês | MEDLINE | ID: mdl-28368112

RESUMO

Epitaxially connected assemblies of nanocrystals (NCs) present an interesting new class of nanomaterial in which confinement of charge carriers is intermediate between that of a quantum dot and a quantum well. Despite impressive advances in the formation of high-fidelity assemblies, predicted collective properties have not yet emerged. A critical knowledge gap toward realizing these properties is the current lack of understanding of and control over the formation of epitaxial interdot bonds connecting the NCs within the assemblies. In this work we demonstrate successive ionic layer absorption and reaction (SILAR) to enhance the interdot bonding within the NC assembly. SILAR treatment improved the fraction of interdot bonds from 82% to 91% and increased their width from 3.1 to 4.0 nm. Absorption spectra and charge transport measurements indicate that the effect of postassembly growth on quantum confinement in this system depends on the composition of the SILAR shell material. Increased NC film conductance following SILAR processing indicates that building and strengthening interdot bonds lead to increased electronic coupling and doping in the assemblies. The postassembly film growth detailed here presents an opportunity to repair structural defects and to tailor the balance of quantum confinement and interdot coupling in epitaxially connected NC assemblies.

9.
ACS Nano ; 11(2): 2284-2303, 2017 02 28.
Artigo em Inglês | MEDLINE | ID: mdl-28178419

RESUMO

Decades of research focused on size and shape control of iron oxide nanoparticles have led to methods of synthesis that afford excellent control over physical size and shape but comparatively poor control over magnetic properties. Popular synthesis methods based on thermal decomposition of organometallic precursors in the absence of oxygen have yielded particles with mixed iron oxide phases, crystal defects, and poorer than expected magnetic properties, including the existence of a thick "magnetically dead layer" experimentally evidenced by a magnetic diameter significantly smaller than the physical diameter. Here, we show how single-crystalline iron oxide nanoparticles with few defects and similar physical and magetic diameter distributions can be obtained by introducing molecular oxygen as one of the reactive species in the thermal decomposition synthesis. This is achieved without the need for any postsynthesis oxidation or thermal annealing. These results address a significant challenge in the synthesis of nanoparticles with predictable magnetic properties and could lead to advances in applications of magnetic nanoparticles.

10.
Proc Natl Acad Sci U S A ; 113(41): 11420-11424, 2016 10 11.
Artigo em Inglês | MEDLINE | ID: mdl-27681627

RESUMO

Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.

11.
Nano Lett ; 16(9): 5714-8, 2016 09 14.
Artigo em Inglês | MEDLINE | ID: mdl-27540863

RESUMO

Epitaxially connected superlattices of self-assembled colloidal quantum dots present a promising route toward exquisite control of electronic structure through precise hierarchical structuring across multiple length scales. Here, we uncover propagation of disorder as an essential feature in these systems, which intimately connects order at the atomic, superlattice, and grain scales. Accessing theoretically predicted exotic electronic states and highly tunable minibands will therefore require detailed understanding of the subtle interplay between local and long-range structure. To that end, we developed analytical methods to quantitatively characterize the propagating disorder in terms of a real paracrystal model and directly observe the dramatic impact of angstrom scale translational disorder on structural correlations at hundreds of nanometers. Using this framework, we discover improved order accompanies increasing sample thickness and identify the substantial effect of small fractions of missing epitaxial bonds on statistical disorder. These results have significant experimental and theoretical implications for the elusive goals of long-range carrier delocalization and true miniband formation.

12.
Nat Mater ; 15(5): 557-63, 2016 05.
Artigo em Inglês | MEDLINE | ID: mdl-26901512

RESUMO

Epitaxial attachment of quantum dots into ordered superlattices enables the synthesis of quasi-two-dimensional materials that theoretically exhibit features such as Dirac cones and topological states, and have major potential for unprecedented optoelectronic devices. Initial studies found that disorder in these structures causes localization of electrons within a few lattice constants, and highlight the critical need for precise structural characterization and systematic assessment of the effects of disorder on transport. Here we fabricated superlattices with the quantum dots registered to within a single atomic bond length (limited by the polydispersity of the quantum dot building blocks), but missing a fraction (20%) of the epitaxial connections. Calculations of the electronic structure including the measured disorder account for the electron localization inferred from transport measurements. The calculations also show that improvement of the epitaxial connections will lead to completely delocalized electrons and may enable the observation of the remarkable properties predicted for these materials.

13.
J Phys Chem B ; 112(42): 13326-34, 2008 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-18826188

RESUMO

The photochemical generation of highly rotationally excited diatomics affords us an intriguing way to study energy relaxation processes in solution. Because excited products involve only a single intramolecular degree of freedom and because their relaxations can lie well outside of the linear-response regime, it may be possible to infer detailed molecular mechanisms for these processes just from transient absorption measurements. In this paper we describe a theoretical study of the rotational relaxation of a new candidate for such measurements, OH radicals. Much as we saw in our previous studies of rotationally hot CN radicals, molecular dynamics simulations of OH relaxation predict that the rotational motion should trigger a structural change in the surrounding solvent, decreasing the rotational friction and allowing the OH to rotate coherently for a dozen rotational periods. The mass distribution in OH, however, gives it a much faster rotational period and significantly different kinematics. These differences end up making it possible to identify the separate molecular events taking place at the onset of the relaxation (an unusual occurrence for a liquid-state process) and to weigh in on what collisions are really like in a liquid.

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